Waveguide junction circulator wherein all modes in each branch arm are evanescent

ABSTRACT

A waveguide junction circulator of the type having a resonant cavity loaded with ferrimagnetic material wherein the coupling ports consist of a length of waveguide having a cutoff frequency which is higher than the operating frequency and wherein said waveguide is tuned to the operating frequency by inserting therein, along the broad wall at predetermined lengths, screws whose capacitive reactance is equal to the conjugate of the imaginary characteristic impedance of the waveguide. In a preferred embodiment, the cutoff frequency of the rectangular waveguide is made tunable and the resulting passband variable by inserting in said waveguide, along the sidewalls, ferrimagnetic strips and subjecting said strips to a variable magnetic field.

United States Patent [72] inventor Rk'llard Flnnie Sltedd BkhopsStortlord, England [21 I Appl. No. 886,639 F 221 Film Dec. :9, 1969 [4S]Patented July 13,197] [73] Assignee International Standard ElectricCorporation New York, N.Y. [32) Priority Mar. 5, I969 [33] Great Britain[31] 1 "83/69 [54] WAVEGUIDE JUNCTION CIRCULATOR WHEREIN ALL MODES INEACH BRANCH ARM ARE EVANESCENT 5 Claims, 8 Drawing Figs.

[52] U.S.Cl. 333/].1, 333/73 [5t] Int. Cl. i. H0lp l/32, l-lOlp 5/12[50} FleldolSearch 33311.!

[56] Relerences Cited UNITED STATES PATENTS 3,492,60l l/l970 Omori333/Ll INDER l l l l FERRIMAGNET CYL OTHER REFERENCES Chart et a]. NewMicrowave Circulators," TRONl(S.Dec. [8 195933311 ELEC- ABSTRACT: Awaveguide junction circulator of the type having a resonant cavityloaded with ferrimagnetic material wherein the coupling ports consist ofa length of waveguide having a cutoff frequency which is higher than theoperating frequency and wherein said waveguide is tuned to the operatingfrequency by inserting therein, along the broad wall at predeterminedlengths, screws whose capacitive reactance is equal to the conjugate ofthe imaginary characteristic impedance of the waveguide. In a preferredembodiment, the cutolf frequency of the rectangular waveguide is madetunable and the resulting passband variable by inserting in saidwaveguide, along the sidewalls, ferrimagnetic strips and subjecting saidstrips to a variable magnetic field.

PATENTEnJuusml 3593.210

H93 Ffiequeng/ 0/12 FERRIMAGA/ r/c I 8 LOADING STRIP I lnvenlor RICHARDF. $KD0 A Home y WAVEGUIDE JUNCTION CIRCULATOR WHEREIN ALL MODES IN EACHBRANCH ARM ARE EV ANESCENT FIELD OI" THE INVENTION This inventionrelates to waveguide junction circulators.

SUM MARY OF THE INVENTION It is an object of the present invention toprovide an improved junction circulator.

According to the invention there is provided a waveguide junctioncirculator of the type having a resonant cavity loaded with aferrimagnetic material, a source of a magnetic field, and a plurality ofports coupled to said cavity wherein at least one of said portscomprises at least one section of waveguide having a cutoff frequencyabove the resonant frequency of said cavity and means for terminatingsaid section length of waveguide with a reactance whose value is equalto the conjugate of the imaginary characteristic impedance of eachsection length of waveguide.

BRIEF DESCRIPTION OF THE DRAWINGS The above-mentioned and other objectsof the invention will become apparent by reference to the followingdescrip tion in conjunction with the accompanying drawings, in which:

FIG. I shows a three-port waveguide junction circulator with ports ofrectangular cross section,

FIG. 2 is a transmission line equivalent circuit of a section of oneport of the circulator of FIG. I in respect of evanescent H waves,

FIG. 3 is a lumped circuit equivalent of the section,

FIG. 4 shows the performance of a junction circulator as shown in FIG.1,

FIG. 5 shows a three-port waveguide junction circulator with ports ofrectangular cross section loaded with ferrimagnetic material,

FIG. 6 shows effective permeability vs. angular frequency fortransversely magnetized ferrite,

FIG. 7 shows a section of waveguide loaded with ferrimagnetic sidewallstrips,

FIG. 8 shows insertion loss vs. frequency for a ferrite loaded sectionof waveguide in cutoff condition with DC magnetic field as a parameter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. I shows a three-portjunction circulator with a central resonant cavity I containingferrimagnetic material 2. The ferrimagnetic material 2 is subjected to aDC magnetic field H m in. the direction indicated to obtain circulationin an anticloclrwise direction.

The three ports 4 of the circulator are identical in construction andoperation. Each port 4 consists of a length 5 of rectangular crosssection waveguide of length 21 with two adjustable capacitive screws 6and 7 on the longitudinal centerline of the upper broad wall of thewaveguide. The Iongitu' dinal spacing between the screws 6 and 7 being I(each screw being at a distance of [/2 from the center point of thelength) with each screw extending into its respective waveguide.

The rectangular cross section (height and width) of each length 5 issuch that at the operating frequency of the circulator, i.e., theresonant frequency of the cavity 1, the cutoff frequency of each length5 is above the operating frequency. For example, at an operatingfrequency of4 GI'IL, each length 5 has typically a height of 0.4 ins.and a width of 0.9 ins.

Assuming electromagnetic energy at the required operating frequency tobe present, in the dominant H mode, at the outer end of the left-hand(inlet) port 4, since the length Sis dimensioned to be beyond cutoff atthis frequency. all modes in the length are evanescent.

A waveguide at frequencies below cutoff exhibits characteristics commonto all nondissipative filter networks in their stopband region. Thecharacteristic impedance which is real In the passband becomes imaginaryin the stopband. The propagation constant which is imaginary in thepassband becomes real in the stopband.

The transmission line analog of a section of length I of the input port4 is shown in FIG. 2 as a line of length I having a positive imaginarycharacteristic impedance jZ and a real propagation constant y Toevanescent H waves therefore the length 1 looks like a pure inductance.The lumped circuit equivalent of FIG. 2 is shown in FIG. 3 as a rrequivalent circuit giving the inductance reactance values as functionsof 2 y, and I.

If the section of waveguide is terminated in a capacitance C such thatthe capacitive reactance X is the conjugate of the inductive reactanceof the section, there will be full energy transfer through the section.The energy transfer is frequency sensitive and the section behaves as abandpass filter.

The passband frequency limits (I, andfz) are given by:

The center frequency, f, occurs at the geometric mean Therefore Thebandwidth is a function of y, and (in the ideal lossless case) as 7,!approaches a due to 1 then tanh ,1 coth ,1, and the bandwidth (frfreduces towards zero.

The inlet port therefore behaves as a two section band-pass filter, inwhich the required value for each section capacitance to provideconjugate matching with the inductance of the respective section isprovided by suitably adjusting the capacitive screws 6 and 7 of theinlet port.

Thus there is full energy transfer through the inlet port to the centralcavity l and subsequent anticlockwise rotation and transmission into theadjacent port. Full energy transfer through this output port occurs bythe same process as that described for the inlet port.

FIG. 4 shows the performance of a waveguide junction circulator as shownin FIG. I, having ports of rectangular cross section with internaldimensions of 0.622 ins. X 0.40 ins., capacitive screws 8 B.A., and aferrite cylinder 0.32 ins. diameter X 0.40ins.

The general functioning of the circulator is conventional, in permittingenergy transfer from one port to an adjacent port only in the directionof rotation determined by the magnetic field applied to theferrimagnetic material. Application of the circulator is alsoconventional such as coupling one port to an aerial, one port to amicrowave radio transmitter, and one port to a microwave radio receiver.

Energy propagation external to the circulator ports may be bypropagating waveguide, in which case a further capacitive matching screwwill be required at each junction between the evanescent waveguide andthe larger dimensioned propagating waveguide. Alternatively, thecirculator may be coupled into a microwave system constructed entirelyin evanescent waveguide.

FIG 5 shows another form of three-port junction circulator, with thesame reference numerals as in FIG. I used to indicate like elements. Asin the circulator of FIG. 1, each port behaves as a two sectionevanescent waveguide band-pass filter tor full energy transmissiontherethrough at the operating frequency.

Each port 4 is constructed of waveguide having height and. widthdimensions such as to be propagating at the operating frequency. Eachport contains loading strips 8 of ferrirnagnetic material, such asferrite or garnet, symmetrically ar-- ranged one at each sidewall of thewaveguide. The ferrimagnetic material strips 8 are subjected to a DCmagnetic field m,-

It has been established that for a transversely magnetized ferrite inrectangular waveguide (TE mode) the cutoff frequency may be controlledby the DC magnetic field. The cutoff frequency can be made higher orlower than the empty waveguide value. This is a consequence of the factthat the ef fective permeability t, of the ferrite can be varied frompositive to negative values by the DC magnetic field as shown in FIG. 6in which m is the cutoff frequency and w, the gyromagnetic resonance forthe infinite ferrite medium. Thus for O the RF field is concentrated inthe ferrite and the effective width of the waveguide is increased,whereas for t, O the RF energy is excluded from the ferrite and theeffective width is reduced. This latter effect is illustrated in FIG. 7.

Thus, in each port ofthe circulator of FIG. 5, the length of waveguideis in the evanescent condition brought about as explained above by theDC magnetic field. This condition is illustrated in H6. 8 and also showshow the cutoff frequency of each port is dependent on the value of DCmagnetic field. The field may be made variable in value when applied byper manent magnet pole pieces by arranging for the pole pieces to bemovable, and when applied by electromagnetic pole pieces by varying thecurrent.

Each port is tunable in frequency by variation in the value of the DCmagnetic field. An increase in field raises the frequency, and areduction in field lowers the frequency.

Since the resonant frequency of the cavity 1 is also frequency variableby its magnetic field, the junction circulator is frequency variable.

The junction circulator may alternatively be constructed with ports ofwaveguide lengths having square or circular cross section.

In the junction circulators of FIGS. 1 and 5, each port may have one,three or more sections each containing a single capacitive screw.

In both the described junction circulators, the capacitive screw orscrews in each section of the ports may be replaced by other forms ofcapacitive obstacle, such as adjustable capacitive diaphragms.

The described junction circulators olTer a fundamentally broaderfrequency band capability than conventional (dispen sive) waveguidejunction circulators because of the lumped element nature of theevanescent mode ports.

I claim:

I. A junction circulator of the type having a resonant cavity loadedwith a ferrimagnetic material, a source of a magnetic field, and aplurality of ports coupled to said cavity wherein at least one of saidports comprises:

at least one section of waveguide having a cutoff frequency above theresonant frequency of said cavity; and

means for terminating said section length of waveguide with a reactancewhose value is equal to the conjugate of the imaginary characteristicimpedance of each section length of waveguide.

2. A junction circulator, according to claim I, having three ports, eachport comprising two contiguous section lengths of waveguide having acutoff frequency above the resonant frequency of said waveguide andwherein said means for terminating each section length includes one ormore capacitive screws mounted on one broad wall of each section lengthof waveguide.

3. A junction circulator, according to claim I, wherein said at leastone section length of waveguide includes a waveguide loaded withferrimagnetic material.

4. A junction circulator, according to claim 3, wherein said waveguideis symmetrically loaded with ferrimagnetic material on each sidewall.

5. A junction circulator, according to claim 3, having three ports, eachport comprising two contiguous section lengths of waveguide and whereinsaid means for terminating each section length includes one or morecapacitive screws mounted on one broad wall of each section length ofwaveguide.

1. A junction circulator of the type having a resonant cavity loadedwith a ferrimagnetic material, a source of a magnetic field, and aplurality of ports coupled to said cavity wherein at least one of saidports comprises: at least one section of waveguide having a cutofffrequency above the resonant frequency of said cavity; and means forterminating said section length of waveguide with a reactance whosevalue is equal to the conjugate of the imaginary characteristicimpedance of each section length of waveguide.
 2. A junction circulator,according to claim 1, having three ports, each port comprisinG twocontiguous section lengths of waveguide having a cutoff frequency abovethe resonant frequency of said waveguide and wherein said means forterminating each section length includes one or more capacitive screwsmounted on one broad wall of each section length of waveguide.
 3. Ajunction circulator, according to claim 1, wherein said at least onesection length of waveguide includes a waveguide loaded withferrimagnetic material.
 4. A junction circulator, according to claim 3,wherein said waveguide is symmetrically loaded with ferrimagneticmaterial on each sidewall.
 5. A junction circulator, according to claim3, having three ports, each port comprising two contiguous sectionlengths of waveguide and wherein said means for terminating each sectionlength includes one or more capacitive screws mounted on one broad wallof each section length of waveguide.